Note: Descriptions are shown in the official language in which they were submitted.
~0~383
me present invention relates to controls for electric power
sources, and more particularly to a control system for substituting one
source for another within an extremely short period of ti~e,
With many types of electrical and electronic apparatus it is
necessary that the apparatus remain in an energized state despite the
failure of a regular power source. For this reason a number of systems
have been devised for producing emergency or standby power in the event of
the failure of the principal power sourceO Such systems as presently
known generally fa~l into two categoriesO
In the first category, which may be termed "uninterruptible"
supply system, a load is supplied directly through the "standb~" source,
which may for instance be an inverter powered by a battery. me principal
or "line" power source constantly charges the battery through a primary
(first) rectifier which supplies power to the inverter for energizing the
load. When a failure of the principal source occurs power continues to
flow from the battery into the inverter, so that no interruption of power
to the load is experienced. Of course, after some period of time the
battery becomes discharged inasmuch as it is no longer receiving charging
current from the outside source. Although the so-called uninterruptible
type of power supply ordinarily performs well~ it is ~meconomical for
many installations inasmuch as the primary rectifier system must be large
enough to supply all of the power needed by a load under all circumstances,
and moreover must be constructed to provide extremely high reliability and
&rability inasmuch as the "standby" system must operate continuously and
if not sufficiently durable can itself be a source of failure.
A second and less expensive approach to providing an alternative
power supply is to provide a battery-operated "standby" system which does
not operate until the principal power source fails. Typically~ a small
charging system is provided to keep the battery of the system fully charged.
A sensor, relay or the like is coupled in circuit with the principal or
~L09~ 33
"line" power source so that when the principal source fails the standby
inverter is immediately energized. Unfortunately~ while the latter approach
is comparatively economical and straightforward, the switchover procedure
which it necessitates causes interruptions in the continuity of power flow
which are~ for some types of loads~ extremely significant.
With the increasing use of computers and other data processing
machinery having electrically-energized memories or registers in which
data is stored~ assuring a continuity of supplied power is extremely
importantO Also of particular interest are lighting systems incorporating
high intensity discharge (hereinafter HID) lamps. Lamps of this type
consideration commonly comprise an envelope filled with a vapor of mercury,
sodium~ or one or more metal halides which ionize in the presence of a
current flow and produce illumination. Since their inception HID lamps
have become increasingly popular, both for their lighting characteristics
and for their high lumens per watt rating, which makes them relatively
economical to operateO
While it is often desirable to use HID lamps for indoor lighting~
and particularly in commercial buildings~ modern lighting codes commonly
require the presence of standby or emergency power sources to assure
continued illumination in the face of power failures.
Unfortunately, while HID lamps are in most respects well suited
for use in populated structures, once they have been extinguished they must
either be allowed to cool for a period of approximately 15 or 20 minutes~
or else an inordinately high voltage is required to relight them. The
voltage required is such as to make it impractical to provide standby
power sources capable of producing the necessary voltageO To make matters
worse, HID lamps will commonly become extinguished within approximately
4 milliseconds after a power supply failure, depending upon the specific
type of lamp and the nature of the vapor. The quenching time of the HID
lamps is far more rapid than the operation of the switching means which are
1090883
conventionally utilized in transferring power from an outside source to
a standby unit. Accordingly, it will be appreciated that it would be highly
desirable to provide a rapid transfer system which maintains the effective
continuity of power flow to a load, obviating any detrimental effects on
the load.
It is therefore an obj'ect of the present invention to provide a
power transfer system which operates substantially faster than any prior
art systemO
It is another object to provide a transfer system which will
effectively maintain the continuity of supplied power while providing
isolation from a defective power source and enabling a localized standby
power source.
Still another object is to provide an improved rapid power
transfer system.
Another object is to provide a rapid transfer system which will
maintain the illumination of HID lamps while decoupling the load from
the principal power supplyO
According to the present invention there is provided a rapid
transfer circuit for disconnecting a load from an AC source including'an
electronic valve having a predetermined holding current, and supplying
power to the load by means of an inverter, comprising: line sensing means
adapted to be coupled to the AC source for deriving an indication of source
voltage; timing means coupled to said line sensing means for producing an
indication of the phase of source voltage; comparison means coupled to said
line sensing means for receiving said source voltage indication and
responsive thereto to produce a transfer signal in response to a drop in
source voltage below a predetermined value; an lnverter adapted to be'
coupled to the load for supplying current thereto; and gating control means
coupled to said comparison means and said timing means and responsive to said
transfer signal for selectively enabling the inverter to operate in phase
~090883
with previously-detected line voltageO
Signals from the timing means allow the standby source to come
on line in synchronism with the previously-detected line voltageO
Preferably a current sensor associated with the electronic valve
means produces a signal indicative of the presence and direction of current
through the valve. me polarity with which the standby power system is
first enabled is controlled as a function of the direction of current
flow through the valve~ so that the standby unit immediately gives rise to
a voltage which opposes the valve current flow~ After the valve i~
commutated the operation of the standby source is determined solely by the
synchronous gating control~ which remains in operation until line power
has been restoredn
The invention will now be described in greater detail with
reference to the accompanying drawings in which:
Figure 1 is an idealized functional diagram of the present
invention, and
Figure 2 is a schematic diagram illustrating a presently preferred
embodiment of the invention~
A load 10, to which it is desired to provide a substantially
uninterruptible flow of power~ is coupled by way of an AC controlling means
12 to a-source of line power 14 which may, for example, represent a distant
power station or the like. ~ine sensing means 16 is coupled to the power
supply lines and ou~puts a signal representative of line voltage to a main
control 18. The main control also receives a signal from a current sensor
20 which indicates the sense of current flowing through the AC controlling
means, which may co=prise a solid-state gated electronic valve. The main
control outputs a GATE signal to the AC controlling means for enabling the
solid-state valve to allow power to flow to load 1OD
A standby power supply 22 is energized by a battery 24, and its
operation is controlled by the main control 180 External circuits 26 of
--4--
1090883
a conventional nature are provided in conjunction with the main control so
that the main control may be operated or disabled by stimuli other than
those derived from the line sensing means or current sensor and may, by way
of example, comprise a simple cut-off switch for manually disabling the
standby power functionO
As is conventional wi~h battery--operated standby power circuits,
maintenance and alarm circuits 28 are provided in conjunction with the stand-
by supply and the battery and serve principally to maintain battery 24 in
a fully charged state, and to give warning, by way of an alarm 30, of a
degradation in the condition of the batteryO Further~ the circuits 28 may
also serve to control or limit current flow from or to battery 240 It
is considered that external circuits 26~ alarm 30, and maintenance and alarm
circuits 28 are of the type ordinarily used in conjunction with the battery-
powered standby supplies, and thus familiar to those skilled in the artO
Inasmuch as such systems are commonly understood and their design straight-
forward, the details of their construction will not be discussedO
Turning to AC controlling means 12, it is recognized that with
some types of line failures it is necessary to decouple the load from the
line before engaging the standby power supplyO Such an instance arises
when a stoppage of power occurs while many other loads are coupled to the
lineO In such a situation, the line appears as a short circuit across load
10~ making it impossible to supply adequate power by means of the standby
power supplyO Similarly when a "brownout" or reduction in voltage occurs,
it is necessary to decouple the load from the line so that the load
supply voltage can be increasedO
Ordinarily, the load and/or the standby power supply is coupled
to the outside source (line) by means of a transformerO Moreover~ rapid
decoupling of the load from the line is best effected by electronic valve
means such as triacs ~silicon controlled bilateral switches) or silicon
controlled rectifiers (hereinafter SCR's) placed in series with the line-
--5--
1090~
energi7.ed transformer windingO Unfortunately, such solid-state electronic
valves exhibit a "holding current" characteristic wherein the valve will
remain conductive until current flow therethrough diminishes to some low
value~ at which time the valve commutates or ceases to conduct. This occurs
in spite of the termination of a gating signal to the deviceO Thus~ when
a power failure occurs the inherent magnetizing current of the coupling
transformer may serve to maintain current flow in the "line" circuit,
preventing the solid-state valve from commutating despite the fact that line
power has been lostO In theory, it is possible to build a workable comm-
utating circuit for the valve in accordance with known techniquesO See~for instance, the SCR Manual, Fifth Edition (1972) published by the
General Electric Company at Auburn, New YorkO
Under normal circumstances, power from a distant AC source 14 is
supplied through lines and A~ controlling maans 12 to a load 10~ Main
control 18 supplies a GATE signal to the AC controlling means 12. Should
the controlling means comprise, for instance, a triac or a pair of opposed
SCR s a continuous GATE signal may be applied to cause the electronic
valves to be responsive to the periodic fluctuations in source voltage so
as to allow AC current to flowO ~ine sensing means 16 applies an indication
of line voltage to the main control 180 mis information is used by the
main control as a timing or synchronizing signal for indicating the desired
phase when a standby power supply 22 is to be brought on lineO A current
sensor 20 supplies information to the main control regarding the instant-
aneous direction and relative magnitude of current flow through the solid-
state valve of AC controlling means 120 The maintenance and alarm circuits
28 continue to monitor the status of battery 24 and assure that the battery
remains in a fully charged conditionO
Upon the occurrence of a fault in power supply 14 an indication
thereof is conveyed to the main control by the line sensing means 160
Inasmuch as the line sensing means produces a manifestation of line voltage,
--6--
08~3
it allows the main control to react to a "brownout" situation wherein line
voltage has diminished significantly below the normal valueO Accordingly,
the main control reacts to the presence of either a total failure of or
decrease in voltage, or a significant shift in phase of line voltageO The
main control then discontinues the gating signal to the AC controlling
means, and energizes the standby power s~pply 220 If the current detector
20 indicates that current above the minimal "holding current" value is
flowing in the solid-state valve of AC controlling means 12~ this inform}
ation takes precedence over synchronization and standby supply 22 is
energized with a polarity for giving rise to a voltage within the AC
controlling means which opposes the flow of current within the solid-state
valveO This effects a rapid quenching or commutation of the valve~ immed-
iately isolating the standby power supply and the load from the power
linesO The co = tation of the valve is sensed by the current sensor 20,
whereupon the main control 18 causes the standby power supply to operate
in synchronism with the voltage previously sensed upon the lines by sensing
means 160
The foregoing activity occurs extremely rapidly~ ordinarily in
less than 4 milliseconds so that the flow of power to load 10 is substan-
tially uninterruptedO In circumstances wherein the polarity necessaryfor commutating the valves of the AC control coincides with the normal
polarity of line voltage, initiation of standby power supply 22 is in the
appropriate polarity and need not be interrupted after commutation of the
valveO When the i~itial polarity which is necessary for commutation does
not coincide with the phase of the previously sensed power si.gnal, the period
required for commu~ation and synchronization of the standby power supply
is so brief as not to constitute a significant interruption in the contin-
uity of power supplied to the load lOo
Turning now to Fig. 29 there is shown in schematic form a presently
preferred embodiment of the inventionO A transformer 32 couples load 10
--7--
~(~91D~33
to lines supplying outside power and to a standby power supply 229 herein
depicted as an inverter including a pair of SCRs 33, 34~ Maintenance and
alarm circuitry 35 are provided and function in the manner described with
respect to Fig. 1 to monitor the operation and charging of a battery 24.
A solid- state switching element, herein shown as a triac 36~
serves to couple a source of AC voltage (not shown) to a winding of trans-
former 32~ The triac serves to decouple the transformer from the lines
which supply power to the system from a distant source~ thereby isolating
load 10 and inverter 22 from the lineO The line sensing means 16, herein
illustrated in Fig. 2 as a line tap~ is coupled to the lines and provides
manifestation of line voltage along with phase and frequency information to
the system. Current detector 20 is coupled in current-sensing relationship
with triac 36 and produces an indication of the sense and relative magnitude
of triac currentO While detector 20 is depicted as including a pair of
opposed diodes 37, 38 across which the voltage drop is monitored~ it will be
recognized by those skilled in the art that many other approaches are avail-
able for monitoring current flow which may be selected as appropriate for
a given systemO
The line top of the line sensing means is coupled to a zero-
crossing detector comprises timing means which outputs a timing signal to aphase-locked loop 42 for causing the loop to produce a periodic signal in
synchronism with line voltageO Inasmuch as zero-crossing detectors are com-
monly used for timing purposes and for triggering various electronic elements,
the specific construction of detector 40 is not depicted~ Any appropriate
detector may be used, preferably one which outputs a train of square wave
pulses whose leading edges coincide with the zero-crossing time of the vol-
tage sensed upon the power supply lines by sensing means 16.
The slew rate of phase-locked loop 42 is such, and its range is
so limited~ that substantial changes in phase in line voltage will not be
traced by the loop. Under normal conditions, however~ the loop ~2 outputs
--8--
10908E~3
a signal such that the signal applied to the phase comparator 47 is in
synchronism with the line voltageO In the illustrated embodiment an output
reference signal at 1200 Hzo is provided to a counting circuit 44 which
effectively divides the input frequency by a factor of 10, the divided signal
being then applied to a bistable counter 46 ~hich in effect divides the
input signal by a factor of 2 and applies it to a phase comparator 47.
Phase-locked loops have long been familiar to circuit designers,
and the construction of an appropriate loop is deemed to be well within the
skill of the routineer in the ar$0 Accordingly, the specifics of phase-
1~ locked loop 42 are not depicted. It is contemplated that an appropriate loopmay easily be constructed of discrete components, or one loop may easily be
constructed of discrete components, or one of the many commercially available
integrated circuit phase-locked loops may be selected for use. A presently
preferred embodiment utilizes an integrated circuit marketed by RCA and
designated as No. CD4046.
me bistable element 46 may comprise an ordinary flip-flop
having a pair of mutually opposed outputs designated Q and Q0 In this manner~
element 46 produces a "toggle" signal which changes state twice for every
cycle of line voltageO Counter 44~ which in a successfully tested embodiment
comprised an RCA integrated circuit NoO CD4017, includes a series of sequen-
tially energized output terminals which are coupled to a summing network 48.
The summing network comprises a group of appropriately-weighted resistive
paths for outputting a signal having a predetermined value for each "count" or
output of the counter 44. In this manner a step-wise generated approximation
of a sine wave is produced and applied to one input terminal of a comparison
means 50, herein depicted as an ordinary comparatorO
me comparator 50 then algebraically combines the digitally-
generated rèference signal produced by summing network 48 with the manifes-
tation of line voltage received from sensing means 160 As long as the line
voltage signal exceeds the reference signal no output is produced by compara-
~9~83
tor 500 However, should line voltage fail, or should it decline below the
value of the reference signal, the comparator will output a transfer signal
to the "set" terminal S of a gating control means comprising flip-flop 52.
In order to allow the system to respond to sudden changes in
line voltage, the manifestation of line voltage is further applied to a
differentiating stage, herein represented at 57. The differentiating stage
(which in some applications may be made integral with comparator 50) responds
to a sudden change in the line voltage to output a transfer signal to the
set terminal of flip-flop 52. The differentiating stage advantageously
displays a threshold response so that no transfer signal is given rise to
by the normal, periodic fluctuations of the AC line voltage.
The timing of flip-flop 52 is controlled by pulses from zero-
crossing detector 40, which are applied to the CLK terminal of the flip-flop.
Further, a time delay module 54, which may comprise ~ simple RC circuit, is
coupled between the set and D terminals of the flip-flop to prevent the
flip-flop from returning to its original state immediately after the termin-
ation of the transfer signalO This precludes the unstable jiggling or oscil-
lating of the standby system in response to repeated perturbations in line
voltageO
As was the case with bistable element 46, flip-flop 52 produces
a pair of mutually opposed output signals or states designated Q and Q.
One of the signals~ herein designated Q~ is processed by a gating circuit
56 and used as a continuous gating signal for triac 36. No particular gating
circuit is necessary for the proper functioning of the system, and it is
contemplated that an appropriate circuit, such as one of those depicted in
the SCR Manual, Fifth Edition, published by the General Electric Company,
may be selected as appropriate.
The opposed Q signal, which arises only in the presence of a
transfer signal from comparison means 50~ is coupled from flip-flop 52 to
an AND gate 58 and to a commutating stage which comprises AND gates 62, 63
- --10_
883
for producing an enabling signal to allow the energization of SCRs 33, 34
of inverter 22. The other input to AND gate 58 is derived from a second
AND gate 60~ As is apparent from the illustration, AND gate 60 is provided
with inverting input terminals~ and accordingly will produce a "1" signal
for matching the Q signal when no enabling signals are present at its inputs,
This corresponds to a condition wherein no current above holding current
value is sensed by current detector 200
One output of the detector is coupled to one of the inverting
inputs of AND gate 60, and to another AND gàte 62 of a commutating stage
by a line designated +Ti which bears a signal in the presence of "positive"
triac current above the holding current value. Similarly, the other inverting
input terminal of AND gate 60 is coupled, along with an input terminal of
another AND gate 63 of the commutating stage, to a second output of detector
20 by a line which carries information designated -Ti, designating "negative~
triac current above the holding current value.
The enabling signal from AND gate 58 is coupled to another
portion of the synchronizing stage comprising a pair of AND gates 64, 65.
The remaining inputs of AND gates 64, 65 are coupled to the Q and Q out-
puts of bistable element 46, respectivelyO In this manner, when enabled by
an output from gate 58, the synchronizing means are caused to "toggle" in
synchronism with previously-detected line voltage.
The output terminals of AND gates 62 and 64 are coupled to an
OR gate 66 so that either output can be transmitted to a common gate circuit
70 which enables SC~ 34. Similarly, the outputs of AND gates 63 and 65
connect to a second OR gate 68 which serves SCR 33 through an appropriate
gating circuit 72. Accordingly, the AND gates 64, 65 comprise synchronizing
means responsive to signals from the timing circuitry for causing inverter
22 to operate in synchronism with the previously-applied line voltageO
AND gates 62~ 63 then comprise commutating means responsive to current
detector 20 for momentarily overriding the synchronizing means to fire
10908~3
whichever of SCRs 33~ 34 necessary to effect the commutation of triac 36.
The operation of the system of Fig. 2 will now be described in
detail, making particular reference to the elements shown therein. Initially,
it is contemplated that load 10~ herein depicted as an HID lamp~ is energi~ed
by AC voltage appearing across the left-hand winding of transformer 32 and
obtained from a distant power source (not shown). Flip~ Mop 52 produces
a continuous gating signal Q which allows triac 36 to be enabled~ so that AC
current flows through the primary unit of transformer 32 and energizes the
loadO ~ine sensing means 16 responds to the applied power to produce a line
voltage replica signal which is applied both to zero-crossing detector 40 and
comparison means 500 Phase-locked loop 42 then outputs a high frequency sig-
nal such that the signal applied to the phase comparator 47 is in synchron-
ism with line voltage and, along with counter 44 and summing network 48,
comprises a reference means which outputs a stepwise reference voltage to
comparator 50. As long as the line voltage replica signal is above the
reference value and no abrupt voltages changes occur no transfer signal is
produced by the comparator, and flip-flop 52 continues to output a ~Ihigh~
Q signal for energiæing triac 36.
At the same time, twice every cycle current detector 20 produces
a signal indicating a flow of current through triac 36. Depending upon
whether the direction of triac current flow is "positive" or "negative~',
the directions being arbitrarily selected, enabling signals are applied to
AND gates 62 or 63~ respectively. The signals indicating the presence of
a greater-than-holding value current are also applied to one of the inverting
inputs of AND gate 60. In this manner AND gate 60 is constrained to output
a logical "0" which prevents AND gate 58 from producing an enabling signal
to allow the triggering of inverter 22.
Should line voltage diminish, or the line fail entirely~ phase-
locked loop 42 will continue to run at the frequency at which it has been
set but the reference voltage produced by the summing network 48 will exceed
-12-
~090~8;~
the sensed voltage derived by the line sensing means 16. In this event
a transfer signal will be outputted by a comparator 50 to the set terminal
S of flip-flop 52, causing the flip-flop to change state.
In a preferred embodiment the system is also sensitive to an un-
toward shift in phase of the line voltage~ wherein the phase of signals from
7ero-crossing detector 40 and bistable element 46 are compared by phase com-
parator 470 If an intolerable discrepancy exists, a transfer signal is out-
putted by the phase comparator 47 to cause flip-flop 52 to change stateO In
this manner gating signal Q will be removed from the triac gate circuit 56 so
that the power lines can be electrically decoupled from the load circuitO
If it were not necessary to maintain a substantially continuous
flow of power to a load~ the foregoing activity would be all that is necess-
ary as far as decoupling the load from the outside power source was con-
cerned. As the polarity of the AC source changes, the resulting back bias
upon the solid-state valve will eventually commutate the valve and open the
line~ This might, however, require a delay of almost a full half cycle, or
approximately 8 milliseconds. Further, and more germane to the subject
at hand, the lagging current drawn by an inductive load and/or coupling
transformer may substantially prolong the time required for commutation,
under some line fault conditions, inasmuch as magnetizing current will
continue to flow through the solid-state valve for a considerable period
after source voltage has reversed. In some cases the magneti~ing current
of a transformer could be as high as two amps in a 5~000 watt system. In
such a case the time required for magnetizing current to decay below the
holding current value is substantially greater than 8 milliseconds, Acc-
ordingly, it will be appreciated that it is necessary to devise a means for
commutating the solid-state valve in order to isolate the load and standby
power source from the faulted supply lineO It is for this reason that a
commutating system mcluding AND gates 62~ 63 and OR gates 66~ 68 is pro-
videdO
-13-
1~90~33
Signals from current detector 20, indicating the presence of
current greater than holding current value in triac 36, serve to prevent
AND gate 58 from producing an enabling signal despite a digital "1" at the
Q output of flip-flop 52. At the same time the digital "1" signal at the Q
output of flip-flop 52 is applied to inputs of AND gates 62, 63 and thus
serves as an enabling signal which allows either OR gate 66 or OR gate 68
to receive a digital "1" depending upon the direction of sensed current
through triac 360 If, for instance, the current flow through triac 36 is in
a direction considered "positive" AND gate 62 will become enabled~ and pass
a digital "1" +Ti signal through OR gate 66 to gating circuit 70. This will
enable SCR 34. me latter is poled so as to induce a voltage in the primary
winding of transformer 32 which opposes the "positive" current sensed in the
triac and commutates the device. In this manner SCR 34 serves to inductively
couple a commutating voltage into the source circuit in response to a detec-
ted~ larger-than-holding value currentO
By the same token if current flow through the triac is sensed to
be in a "negative" direction a -Ti signal is produced by the current detec-
tor. As before~ this serves to inhibit synchroni7ing signals by preventing
AND gate 58 from being energi7ed. The -Ti slgnal is applied to commutating
AND gate 63, (enabled due to the state of flip-flop 52) and through OR gate
68 to gating circuit 72. This effects gating of SCR 33~ which is poled in
the opposite direction to SCR 340 Current flow from inverter 22 then serves
to indu~e a voltage in the source-coupled primary winding of transformer
32 which opposes the previously sensed, negative-going triac current. In
the foregoing manner commutation is provided by inverter 22 in response to
the presence of current in the solid-state valve.
When the valve 36 is fully commutated transformer 32 is effec-
tively decoupled from the outside power source and the lines extending there-
from. Since current flow has ceased, or has at least fallen below the
holding current value~ the signals from current detector 20 cease and no
-14-
~090~83
further outputs are produced by AND gates 62, 63. Further, the absence of
signals from current detector 20 cause in~ibiting AND gate 60 to return to
a "high" output which, in combination with "high" output of flip-flop 52
causes AND gate 58 to produce an enabling signal to synchronizing ~ND gates
64, 65. The output of the latter then commence to toggle in accordance with
the operation of bistable means 46 and in this manner effect the alternate
firing of SCRs 33 and 34O The latter SCRs will continue to alternate in
synchronism with the previously-sensed line voltage due to the continuing
oscillatory signal outputted by phase locked loop 42.
When a properly phased source voltage next appears~ comparison
means 50 returns to its original state and ceases to apply a transfer signal
to flip-flop 52. After the passage of a predetermined period of time, as
determined by the value of the co~ponents constituting time delay 54, the
gating control means reinstates the gating signal to triac 36 at a line
voltage zero crossing, and terminates gating signals to the inverter. Th~s
is effected by a reversal in state of flip-flop 52 so that the Q signal is
now "high" while the Q signal is "low". In this manner a continuous gating
signal is once again applied to enable triac 36, while at the same time
AND gate 58 ceases to produce an enabling signal so that the toggle action
of bistable element 46 is no longer applied to gate circuits 70~ 72. Fin-
ally, since the Q signal of flip-flop 52 has returned to a "low" state,
AND gates 62, 63 are disabled so that signals indicating the presence of
current through the triac are no longer passed to the SCR gate circuitsO
It will now be appreciated that the present invention provides a system for
rapidly transferring supplied power from an outside voltage supply to a
standby supply, such as an inverter, without the need for waiting until
current in the source circuit has decayed below a holding current valueO
The present system thus automatically provides immediate commutation of a
valve in the source circuit in order to decouple the source from the load
and standby s~stem, and at the same ~ime insures that the standby system will
come "on line" in synchronism with the previously existing supply voltage.
-15-
